1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine and the like, and more particularly it relates to a technique for controlling a convey means when a sheet material on which an image was formed in an image forming portion is surface-reversed or turned over by using a re-supply convey path.
2. Related Background Art
In some conventional copying machines in which images can be formed on both surfaces of a sheet material (both-face copy), there are provided a re-supply convey path branched from a convey path at a downstream side of an image forming means disposed within a main body and connected to a portion at an upstream side of the image forming means, and an intermediate tray disposed in the re-supply convey path.
A plurality of sheet materials having one surface (first surface) on which an image was formed are temporarily stacked on the intermediate tray, and, thereafter, each sheet material is re-supplied from the intermediate tray to the image forming means through the re-supply convey path in order to form an image on the same first surface or the other surface (second surface) of the sheet material.
If the sheet material curled due to the image formation on the first surface or the fixing of the image to the first surface is rested on the intermediate tray, the poor conveyance, poor stacking of sheet materials onto the intermediate tray and/or poor sheet material re-supply will occur. Further, if the curled sheet material is re-supplied to the image forming means, image quality will be worsened or sheet material jam will occur.
To avoid the above-mentioned drawbacks, normally, a curl removing means or a curling means for curling the sheet material in an opposite direction is provided in front of the intermediate tray.
Now, a concrete construction of a conventional copying machine 101 having the above-mentioned arrangement will be briefly described with reference to FIG. 5. In FIG. 5, sheet materials S' are separated and supplied one by one from a cassette 111 by means of a pick-up roller 110, and the separated sheet material is supplied to an image forming portion 112 through a vertical convey path 109. After an image is formed on the sheet material in the image forming portion 112, the sheet material is sent to a fixing device 114, where the image is fixed to the sheet material. Thereafter, the sheet material is conveyed to a surface-reverse/discharge portion 115, where a travelling direction of the sheet material is switched in dependence upon whether the sheet material is normally discharged or both-face copy is effected regarding the sheet material.
That is to say, when the sheet material S' is normally discharged, the sheet material is conveyed through the surface-reverse/discharge portion 115 in a direction shown by the arrow A101 to be discharged onto a sheet discharge tray 116. A flapper 115a can be rotated to switch convey paths.
When the both-face copy is effected, the sheet material is conveyed in a direction shown by the arrow A102 to be introduced into a re-supply convey path R101 (shown by the dot and chain line in FIG. 5), where the curl formed in the sheet material is removed by a first curl removing means 120 and a second curl removing means 121. Thereafter, the sheet material is rested on an intermediate tray 122. When an image is to be formed on the second surface of the sheet material, the sheet materials stacked on the intermediate tray are separated one by one by means of a re-supply means (pick-up roller 123 and separation means 124), and the separated sheet material is passed through a lower convey portion 126 via a pair of convey rollers 125. Then, the sheet material is re-supplied, through the vertical convey path 109, to the image forming portion 112, where an image is formed on the second surface of the sheet material. Then, the sheet material is sent to the fixing device 114, where the image is fixed to the second surface of the sheet material. Thereafter, the sheet material is discharged through the surface-reverse/discharge portion 115.
The first and second curl removing means 120, 121 will be explained with reference to FIGS. 6 and 7. The first curl removing means 120 shown in FIG. 6 comprises an iron roller 120a and a sponge roller 120b arranged in such a manner that the iron roller 120a is penetrated into the sponge roller 120b to deform a part of the latter. When the sheet material S' passes through a nip between the iron roller 120a and the sponge roller 120b, the sheet material S' is urged against the iron roller 120a to be curled along the iron roller 120a, thereby removing the curl directing toward a direction opposite to the iron roller 120a.
The second curl removing means 121 comprises a lower roller 121a having a through shaft, a holder 121b pivotally mounted on a pivot shaft 121c and biased toward the lower roller 121a, and two rollers 121d, 121e rotatably supported by the holder 121b. When the sheet material S' passes through the second curl removing means, the sheet material is urged against the lower roller 121a by the rollers 121d, 121e, thereby curling the sheet material along the lower roller 121a (removing the curl directing toward a direction opposite to the lower roller 121a). In order to improve the stacking of the sheet materials onto the intermediate tray 122, flanges each having a height of about 0.5 mm are provided on both ends of the lower roller 121a, thereby enhancing sheet material discharging ability.
The copying machine 101 further includes a surface reversing function for discharging the sheet material S' onto the discharge tray 116 while surface-reversing the sheet material. The surface reversing function is required for discharging and arranging the sheet material S' in a copying sequence by surface-reversing the sheet material S' (on an upper surface of which the image was formed in the image forming portion 112) and then discharging the sheet material onto the discharge tray 116.
Explaining the surface reversing function with reference to FIG. 5, the sheet material S' to which the image was fixed is directed to the re-supply convey path R101 in the direction A102 by the flapper 115a of the surface-reverse/discharge portion 115. At the same time when a trail end of the sheet material S' passes through a lower end of the flapper 115a (this fact is detected by a sensor (not shown)), the first and second curl removing means 120, 121 are rotated reversely to convey the sheet material in an opposite or reverse direction shown by the arrow A103, thereby discharging the sheet material onto the discharge tray 116.
However, the above-mentioned conventional surface reversing mechanism utilizing the curl removing means has the following two problems.
A first problem relates to a wrinkle generated in the sheet material when the sheet material is being conveyed. More specifically, since the sheet material is conveyed in the normal and reverse directions by the curl removing rollers while pinching the sheet material between the curl removing rollers, the greater stress is applied to the sheet material than that in the case where the sheet material is conveyed by normal rollers (not removing the curl), with the result that the wrinkle is apt to be formed in the sheet material. This is true particularly when a thin sheet material having low rigidity or a large sized sheet material (which is pinched between the curl removing rollers for a long time) is conveyed.
A second problem relates to reduction of productivity of the copying machine (during the surface-reverse and discharge of the sheet material) due to the longer time for surface-reversing the sheet material, i.e., reduction of the number of sheet materials to be discharged per unit time.
Explaining this problem with reference to FIG. 8, a distance between the rollers (convey means) must be selected to be smaller than a size of a smallest sheet material to be conveyed. When it is assumed that the smallest sheet material is a statement sheet, the distance between the rollers must be smaller than 139.5 mm (length of the statement sheet). When convey means are arranged as shown in FIG. 8 in accordance with the above conditions, in a case where pairs of convey rollers 117 and 118 are disposed on both sides of the flapper 115a, a distance between the pair of convey rollers 117 and the first curl removing means 120 becomes 110 mm. Further, a distance between the first curl removing means 120 and the second curl removing means 121 is also selected to become smaller than the length of the statement sheet. Incidentally, a distance between the lower end of the flapper 115a and the first curl removing means 120 is selected to 20 mm.
With this arrangement, when a plurality of sheet materials having A4 size are surface-reversed and discharged, a first sheet material is conveyed in the normal direction until a trail end of the first sheet material leaves the lower end of the flapper 115a and reaches a position in front of the first curl removing means 120 by 10 mm.
When the trail end of the sheet material is detected by the sensor (not shown) at this position, the conveyance of the sheet material in the reverse direction is started, with the result that the sheet material is conveyed in the direction A103 by the first and second curl removing means 120, 121.
Accordingly, a tip end of a second sheet material must not reach the first curl removing means 120 until the trail end of the first sheet material leaves the first curl removing means 120.
Thus, the longer a distance L' (200 mm in case of A4 size sheet material) between the convey means for surface-reversing the sheet material (first curl removing means 120, in this case) and the tip end of the sheet material the longer a distance between the first and second sheet materials, with the result that the number of sheet materials discharged per unit time is decreased (reduction of productivity).
In the conventional techniques, since the arrangement of the convey means is limited by the smallest size sheet material to be conveyed, the distance L' cannot be decreased, thereby resulting in the reduction of productivity during the surface-reversing and discharging.